1. Field of the Invention
The present invention relates generally to an electromechanical system, such as an electric motor driven by an inverter, and a method for constructing and using the system. The system of the invention is designed to minimize rotor voltage and to thereby minimize bearing voltage and bearing current discharges.
2. Description of the Related Art
Virtually all rotating motors, generators, etc. to some degree develop either rotor shaft to ground voltage (Vrg), or shaft end-to-end voltage and resulting bearing currents (Ib). Shaft end-to-end voltage is typically the result of internal electromagnetic induction effects. External sources are a primary cause of Vrg via electrostatic coupling. Modem voltage source inverters, such as pulse width modulation (PWM) inverters, are among those external sources which may be electrostatically coupled and thereby serve as sources of Vrg. The resulting Vrg gives rise to electrostatically induced Ib which is a growing industry concern since such currents ultimately result in mechanical damage to the bearing.
In a typical electrostatic coupling situation involving an external source, the rotor shaft voltage buildup occurs when the bearing assembly, comprising the rolling elements, the lubricant, and the bearing races, forms a capacitor (hereinafter a bearing capacitor), thereby providing a charging mechanism for the rotor shaft. The bearing capacitor forms as the rolling elements ride the dielectric lubricant around the races and become separated from the races by this dielectric film. Once the bearing capacitor forms, the external source can capacitively charge the rotor shaft to a voltage in excess of the lubricant""s electric field breakdown intensity. This ultimately leads to an arcing discharge of current across the bearing, pitting the bearing and increasing mechanical wear.
Typically these bearing current events take the form of arcing discharge current pulses commonly referred to as Electric Discharge Machining (EDM) which occur due to capacitive discharge breakdowns in high-resistivity grease. These EDM events can occur when the bearing capacitor voltage, charged by the rotor shaft voltage, becomes high enough to break down the lubricant and a high current discharge flows through the charged oil film capacitor. This occurs when race-to-bearing asperity contacts come close, thereby increasing the oil film electric field and leading to breakdown and resulting high discharge currents. These currents in turn create localized elevated temperatures of the races and can lead to molten pits and fluting of the outer race which in turn can lead to bearing surface damage and, ultimately, bearing failure. Specifically, EDM events can occur when Vrg exceeds a critical bearing threshold voltage (Vth), breaking down the insulating layer of lubricant. These problems are often exacerbated in those cases where PWM inverters are present because PWM inverters can produce higher EDM currents than those observed during sine wave operation due to their application of high common mode voltage and thereby high Vrg.
In designing to minimize the incidence of EDM events, the magnitude of the bearing voltage can be modeled as a capacitor divider circuit with the divider ratio equal to a Bearing Voltage Ratio (BVR). The BVR can be used to evaluate the potential for shaft voltages and bearing currents and to design accordingly. The BVR is equal to Vrg/Vcm where Vcm is equal to the stator common mode voltage. Alternately, BVR can be calculated as Csr/(Csr+Cb+Crf) where Csr equals stator to rotor capacitance, Cb equals bearing capacitance, and Crf equals rotor to frame (rotor to ground) capacitance.
Of the solutions which have been attempted to either minimize Vrg or to reduce EDM, none have been completely successful. One such solution involves attaching a shaft grounding brush to the rotor shaft to bleed off Vrg by creating a short circuit in the system. This approach, however, requires a low resistance contact between the brush and the rotor. In practice such a contact is difficult to maintain due to brush wear, contamination, and surface oxidation.
Another solution to Vrg buildup and the resulting bearing currents is to use conductive grease as opposed to conventional lubricants. The conductive grease is formed by suspending metallic particles in the grease. Experience with conductive grease however indicates that such greases decrease bearing life dramatically.
Another solution to reducing Vrg buildup has been to apply an insulating layer to the rotor shaft or bearing surfaces forming an additional capacitance in series with Cb. However, this approach has raised thermal problems as rotor heat must now traverse this additional insulating layer before reaching the frame.
Yet another approach has been to use an electrostatic, or Faraday, shield inserted between a radiating source and a shielded area to diminish Csr, thereby reducing the coupling between the stator and the rotor. Use of such a shield is logistically difficult however because it must be inserted into the airgap of a motor without short circuiting the stator laminations, degrading the stator insulation system, or bridging the airgap across its entire length.
Certain aspects commensurate in scope with the originally claimed invention are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be set forth below.
In accordance with one aspect of the present invention, there is provided a system for reducing common mode voltage on the rotor shaft of a motor. The system comprises at least a rotor suspended within a frame by a plurality of bearings and a mechanism by which the capacitance between the rotor and the frame is increased, e.g. by increasing the surface area presented between the rotor shaft and the grounded surfaces. The increase in rotor to frame capacitance serves to reduce the voltage on the rotor shaft and thereby to minimize the incidence of harmful bearing current discharges. Dielectric materials disposed adjacent to the surface area enhancement may be used to further increase the rotor to frame capacitance. Ionization of the dielectric material may be done to create a continuous path for current, allowing the shaft to ground voltage to be reduced to near zero.
In accordance with another aspect of the present invention, there is provided a method for reducing detrimental bearing current discharges. The method comprises increasing rotor to frame capacitance by increasing the surface area presented between the rotor shaft and frame of an electromechanical device. The increase in rotor to ground capacitance acts to reduce rotor to frame voltage and to thereby minimize the incidence of bearing current discharges.